Before Big Bang: Light Shed on "Previous Universe"

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This is one reason why researchers have been toiling for decades to unite two main branches of physics—gravity and quantum mechanics.

Gravity rules on cosmic scales, while quantum mechanics dictates the behavior of tiny particles like electrons and quarks.

While each theory has been wildly successful, they remain contradictory.

Uniting these two branches of physics would peel back time further and allow scientists to figure out exactly what the big bang was.

But creating such a "theory of everything" has been a longstanding and difficult goal that has stumped every physicist who has attempted it, including Albert Einstein.

Big Bounce, Not Big Bang

Bojowald used a leading approach to this quandary known as loop quantum gravity, a competitor to the more popular approach known as string theory.

Both theories are still incomplete and unproven, and each suggests very strange ideas about the fundamental nature of the universe.

In loop quantum gravity, for instance, space and time are not smooth and continuous but rather divided up into tiny chunks.

In this mathematical approach, everything is jerky and blocky—although on such a tiny scale that it doesn't affect daily life.

Nothing can occupy a space smaller than the smallest chunk of space, and nothing can happen any faster than this shortest moment of time.

This implies that the universe could never shrink down beyond a certain size. So when it was at its most compact, where did that tiny ball of energy and matter come from?

It could have come from the universe before our own, Bojowald argues. Unlike our expanding universe, this earlier universe was contracting back toward a point, he says.

When it reached its most compact, it hit the barrier dictated by loop quantum gravity. Then it "bounced back" outward, forming a new, expanding universe.

Certain Uncertainty

So if our universe came from an earlier universe, it's natural to wonder what that ancestral universe was like.

But there's a problem: Quantum physics must have played a key role in the hot, dense state around the time of the "big bounce."

Things behave very oddly in the quantum world. An object that appears to be in one spot when you first glimpse it can be in another spot when you look again.

This jumpiness, known as uncertainty, is built into quantum physics. Building better measuring devices won't get around it.

If the whole universe suffered from these jitters, "it could be impossible to have life," Bojowald said.

In our universe, however, such weirdness only happens on very, very tiny scales.

But what about the universe that came before us?

When the universe goes through a big bounce, Bojowald showed, the amount of uncertainty before and after the bounce have little relation to each other.

So there's a veil that screens out much of what we would want to know about the earlier universe.

This also implies that a universe is never the same before and after a bounce.

"The eternal recurrence of absolutely identical universes would seem to be prevented by the ... cosmic forgetfulness," Bojowald said. (Related: "Universe Reborn Endlessly in New Model of the Cosmos" [April 25, 2002].)

Even that kind of cycle might be coming to an end, since scientists now believe that the universe is expanding faster every day, not slowing down as would be expected. So a re-contraction seems extremely unlikely under our current understanding.

Question of Accuracy

Whether Bojowald's model is believable or not, however, depends on whether the version of loop quantum gravity that he used is accurate.

Thomas Thiemann, of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, called Bojowald's approach a "drastic simplification."

But it may turn out to be fairly accurate anyway, Thiemann said.

If so, then it is "the cleanest derivation of a pre-big bang scenario that any physical theory has delivered so far," he added.

It's "much cleaner than in string-theory-inspired models."

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